1. Field of the Invention
This invention relates to power supplies and in particular to a constant frequency current mode switching regulator power supply with an improved regulator system which eliminates oscillations in the widths of the pulses from the power switch in the power supply without the use of any extra feedback loops.
2. Description of the Prior Art
Most control systems for switching power supplies utilize a single control loop that compares the DC output voltage (or current) from the power supply to a fixed reference voltage. Any difference or "error" between the two voltages is amplified and used to directly control the pulse width (i.e., "on time") of the power switch in the power supply in a negative feedback arrangement. This produces a regulated (fixed) output voltage or current. This control method has the following disadvantages:
1. Slow loop response to load transients;
2. Slow loop response to input voltage changes;
3. Allows certain imbalances in push-pull power switch circuits used in the power supply to cause power switch failure due to excessive current caused by transformer saturation.
FIG. 1 illustrates a typical prior art power supply of this type which is sometimes called a "buck" regulator. In the structure of FIG. 1, the output voltage on lead 12 from the power supply (often called a "converter") is sent to error amplifier 15 on the inverting input lead 17a of error amplifier 15. A signal proportional to the output current is sent to error amplifier 15 in the case of a constant current power supply. Voltage reference 18 is applied to the noninverting input lead 17b of error amplifier 15. The output signal on lead 16 from error amplifier 15 reflects the difference between the voltage on output lead 12 and the reference voltage 18 and controls both the duty cycle and width of the pulses produced by pulse width modulator 13 which modulates the input voltage 11. Choke 14 operates in conjunction with capacitor 19 to smooth and average the output pulses from modulator 13.
Improvements over single control loop supplies of the type shown in FIG. 1 have previously been made by adding a second current source loop inside the main voltage loop. FIG. 2 illustrates a typical prior art circuit of this type. The circuit of FIG. 2 includes error amplifier 26 and current sensor 27 as a second loop 30 inside the main voltage loop described above in conjunction with FIG. 1. The error voltage from error amplifier 25 (corresponding to error amplifier 15 in FIG. 1) controls the current source loop 30 which in turn controls the peak switch current from power switch 23 on a cycle by cycle basis. The system including the second loop 30 exhibits much faster response to input and output voltage changes and output current changes, than does the system of FIG. 1. When the power switch 23 is configured as a push-pull circuit, loop 30 corrects for transformer imbalances such as differences in switch transistor storage times, and for noise and load transients which can cause transformer flux saturation and thus excessive DC current in the switching transistors.
Although the addition of the second current source loop 30 as shown in FIG. 2 improves response speed and balance of the power supply, it adds instability to the pulse widths of the pulses from power switch 23 at greater than 50% switch duty cycles. The duty cycle is the ratio of the on-time of power switch 23 to the total period of the pulses from switch 23. This instability appears as subharmonic oscillations of the pulse widths at a frequency lower than the switching frequency. These oscillations alternately narrow and widen the widths of the pulses from power switch 23. This instability may be stopped by the addition of an artificial ramp to the "switch current sense" waveforms generated by switch current sense circuit 27. This ramp is difficult to generate because it ideally should change magnitude with output load if optimum performance of the power supply over a wide load range is required. The oscillation, which occurs at a submultiple of the switching frequency, is a predictable result of using "clocked-on" controllers (i.e., turning on power switch 23 regularly at a fixed frequency), turning off power switch 23 when the current from switch 23 reaches a predetermined level, with duty cycles greater than fifty percent (50%).
Additional prior art is disclosed in Small, U.S. Pat. No. 4,616,301, issued Oct. 7, 1986. Small discloses a power supply which eliminates the potential for oscillation by not utilizing a "clocked-on" controller, but rather a semi-fixed off time controller. This is accomplished utilizing an industry standard part number TL494 control integrated circuit.
In accordance with Small's device, a third loop is added to the structure of FIG. 2 to hold substantially constant over the long term but to allow to vary over the short term the frequency of the power switch 23 (which produces a pulse width modulated output signal from a DC input signal) by controlling "off-time".
Thus the prior art of Small detects average frequency and changes the off time until the frequency is correct, by using a feedback loop. Small's circuit has at least one disadvantage in that it is very complex due to the frequency control circuit having an active feedback loop. Switching power supply regulators operate in a high electrical noise environment due to the switching action of the power switch transistors. A complex regulator having large wiring loops has the major disadvantage that the wiring loops pick up the electrical noise and hence such a regulator may not operate as effectively as is desirable.